This invention relates to method and apparatus for laying underground cable and more particularly to pulling long runs of fiber optic cable through underground conduit.
Fiber optic cable is composed of a bundle of long, thin fibers of glass, plastic or other transparent material, closed within a protective sheath. Encoded light pulses carrying audio and video signals are sent through the fiber much like electric current travels along a wire. The advantage of fiber optic cable over conventional cable lies in its transmission characteristics. Because of the fiber's thinness and superior attenuation characteristics, a fiber optic cable can carry a much higher rate of information over many more channels than a comparably sized wire cable.
However, fiber optic cable is more difficult to lay than conventional cable. It lacks the tensional strength of conventional wire cable and will fracture at a much lower pulling tension. Furthermore, because of its construction, fiber optic cable is relatively inflexible. Typically, the fibers are bundled in a spiral fashion around a stiff steel support wire within a hard plastic protective sheath. Bending of the cable beyond a limited range can break the fibers within.
Because of these material drawbacks, conventional pulling methods and apparatus have proven inadequate for pulling more than a relatively short length of fiber optic cable through an underground conduit. These methods usually comprise placing a single winch at the conduit exit, passing a pull rope attached to the fiber optic cable through the conduit to the winch, and operating the winch to pull the rope and cable through the conduit until the rope is completely wound on the winch and the cable reaches the conduit exit. Pulling a cable in such a manner requires considerable tension to overcome the frictional drag of the cable along the conduit surface. The winch pulling tension necessary to overcome this drag quickly increases as the length of pull increases. Typically, no more than 2,000 feet of cable can be pulled before the winch tension exceeds the cable's tensional limit. (However, the length of pull varies with the condition of the conduit.) In contrast, many times that length of wire cable can be pulled without the cable breaking.
The extra pulling requires more time and manpower in moving the winches and setting up the apparatus. Moreover, connecting the relatively short lengths of fiber optic cable adds substantial additional cost to installation of the cable. Each connection demands expensive and time-consuming splicing. The extra splicing in turn creates resistance to the transmitted light pulses which must be overcome by the installation of additional signal repeaters along the cable to boost the signal strength.
To increase the maximum continuous length of fiber optic cable which may be pulled, several techniques have been developed. In one approach., a tensiometer is incorporated into the conventional winch to limit the pulling torque of the winch to an amount below the tensional strength of the fiber optic cable. The winch ceases to pull if the tension needed to pull the cable through the conduit exceeds the preset torque limit of the winch. If the cable is between conduit access points at the time the winch stops, it is withdrawn until its end rests at an access point. A new section of cable is then inserted and the process repeated. Although this technique assures the winch will not pull the cable with excessive tension, it is little more than a fine tuning of the conventional pulling method and does.not significantly increase the pulling length of cable.
A second technique doubles the continuous length of cable that can be pulled by pulling the cable from both ends of its storage spool, one end in each direction. With the spool positioned at an intermediate access point in the conduit, one end of the cable is pulled in one direction and the other end is pulled in the opposite direction. Under typical conditions, the length of pull is about 4,000 feet.
A third approach encompasses a series of pulls and stores which further increase the pulling length. The cable is first pulled from its storage spool at an intermediate conduit access point 2,000 feet from an entry point of access to the conduit. Rather than stopping at that point where the cable reaches the winch, however, the cable is manually pulled out of the conduit until an additional 2,000 feet are pulled through the conduit. As the cable emerges from the conduit, it is stored on the ground in a FIG. 8 pattern sufficiently sized to avoid unduly bending the cable. After 2,000 feet are stored aboveground, the cable end is then reinserted into the conduit, and the stored cable is pulled to another access point. At the spool, a third 2,000-foot length is spooled off into another FIG. 8 pattern on the ground. The cable is then severed from the spool, its end inserted into the conduit, and the stored cable is pulled in the opposite direction to a third access point. In this way a 6,000-foot run of fiber optic cable can be pulled into a conduit with splicing.
These methods of pulling fiber optic cable, although improvements over the conventional pulling method, can lay but relatively short lengths of cable. Only with the third approach is it possible to pull significantly longer lengths, but the labor cost of handling, storing, and pulling soon outweigh the benefit of an additional increment of length. And all these methods still require significantly more splices and signal repeaters than are required in pulling wire cable.
Accordingly, the need remains for a method and apparatus for pulling long runs of fiber optic cable quickly and efficiently.